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It is now common practice in industrial applications of digital electronics for plant interface units to be geographically distributed and connected to a central point by a time-division multiplexed serial link. As in all such links, some means must be provided for each interface unit to send or accept data only in its allotted time, and the paper describes an arrangement which minimises the time consumed by this addressing function without sacrificing reliability in noisy environments. It entails the central station sending one bit of address information in each unit's time slot, in such a way that the current n-bit address is formed by the n most recently received address bits. Each unit can verify this process. The method is suitable for applications, in which many remote units are scanned sequentially to exchange data with a central station; for example, in datalogging.